Страницы: 965-993
Аннотация
We offer a boreal zonal standard scale providing an effective panboreal correlation and current geological dating of Mesozoic boreal sediments. The need for establishment of a boreal standard is brought about by the fact that most boreal stratigraphic sections cannot be correlated directly, zone by zone, with Mesozoic type sections located in Western Europe. The standard section of boreal Mesozoic represents the most complete sequence of mollusk (mainly ammonite) zones known on the territory of the boreal realm. It is compiled from fragments of zonal sequences established in more than 30 Triassic, Jurassic, and Cretaceous sections of Northern Eurasia, North America, and Greenland. The boreal standard of the Triassic system comprises 36 ammonoid zones and 2 bivalve zones; Jurassic, 70 ammonite zones; Cretaceous, 37 ammonite zones, subzones, and beds, 6 bivalve zones, and 11 belemnite zones and subzones. Siberia and Northeastern Asia provide stratigraphic sections, many of which are the best in the boreal realm regarding their stratigraphic completeness and detailed zonal subdivision. They include the type section of the Olenekian and one of the most complete boreal sections of the Induan stage. Infrazonal scales are constructed for both Induan and Olenekian stages as well as for the Middle Anisian and Upper Anisian. Sequences of ammonite zones from Hettangian, Sinemurian, Toarcian, and Upper Pliensbachian sections, located in Northeastern Asia, as well as the Lower Oxfordian substage from North Siberia and Kimmeridgian stage of the Subpolar Urals are chosen as boreal standards. The ammonite zonal scale of the Upper Volgian substage in the north of Siberia is more complete than that of the Gorodishche type section on the Volga. The most complete and continuous successions of ammonite zones in boreal Berriasian and Valanginian are established in North Siberia. They are adopted as a composite boreal standard. Studies of the North-Siberian Upper Cretaceous sections in a stratigraphic range from the Upper Cenomanian to the Santonian/Campanian boundary revealed analogs of all East- and West-European inoceramid zones. In addition to ammonoid zones for all the Mesozoic systems in Siberia and Northeastern Asia we used the same sections to compile independent parallel zonal scales on parastratigraphic groups of fauna and flora. We also compiled an almost "through" scale, based on bivalves, and some of its intervals entered the boreal standard (Upper Jurassic, Upper Cretaceous). We offer a zonation on nautiloids for the entire Triassic and zonations on conodonts and foraminifers for some other intervals. A set of scales were developed for the Jurassic interval: on belemnites, foraminifers, ostracods, dinocysts, spores, and pollen. The Cretaceous was characterized with dinocyst and foraminifer scales. Taken together, these scales may ensure close correlations, mainly regional. However, many of them, like the zonal bivalve-based scale, allow direct panboreal and even boreal-peritethyan correlations for the Upper Triassic, Lower Liassic, Upper Jurassic, Neocomian, and Upper Cretaceous. The proposed boreal Mesozoic standard should be improved. This is particularly true for a greater part of the zonal scale of the Cretaceous system. Nevertheless, even in its present version it may be used for improvement of panboreal and boreal-tethyan correlations, as well as for current stratigraphic studies of boreal sections in particular regions.
